April 1 (Reuters) - Oregon's biggest city on Wednesday banned the use of an insecticide on city lands blamed by conservationists as a factor in the decline of honey bees in recent years.

Despite protests from farmers who argued the insecticide was crucial for crop production, the Portland City Commission voted unanimously to immediately suspend use of products that contain neonicotinoids.

Such pesticides are widely used on crops and on plants as well as trees in gardens, parks and commercial nurseries.

Portland brings to at least eight the number of U.S. municipalities, including Seattle and Spokane in neighboring Washington state, that have banned the chemicals amid what conservationists say is mounting evidence the insecticide is a culprit in the decline of bees and other pollinating insects.

Portland Commissioner Amanda Fritz successfully sought approval of the measure on Wednesday as a public health issue requiring emergency action that would immediately outlaw use of neonicotinoids in such areas as municipal parks, streets and gardens.

"I think we're doing another good thing for the city of Portland, Oregon ... and maybe the entire world," Fritz said.

Opponents like Oregonians for Food and Shelter, a coalition of farmers, foresters and other pesticide users, said findings by some scientists suggesting honey bees have been severely harmed by the insecticide have been refuted by other researchers.

Scott Dahlman, the group's policy director, said the decision by Portland leaders was based on "fear and ideology" rather than sound science about bees and other pollinators, which are vital for food production.

"Farmers have a huge investment in honey bees but they also need insecticides to protect their crops from destructive pests," he said.

Aimee Code, pesticide program coordinator for the Xerces Society for Invertebrate Conservation, said momentum was building among local governments to prohibit use of such chemicals even as the U.S. Environmental Protection Agency mulls restricting or curtailing their use.

Eugene, Oregon, and Shorewood, Minnesota, are among eight municipalities that have passed bans similar to the one in Portland, said Code.

The federal Fish and Wildlife Service is to prohibit neonicotinoid use at national wildlife refuges by next January.

The agency found that the insecticide, which is taken up by plants through roots and leaves, was not preferred because it could be broadly distributed and potentially affect "a broad spectrum of non-target species." (Editing by Cynthia Johnston and Sandra Maler)